One example of a hazardous area, in which workers are possibly exposed to pyrophoric conditions, involves the reconditioning of vessels in the petroleum refining, petrochemical and associated industries. Within these industries catalytic reactors are used to convert petroleum or gas feed products into multiple products, such as gasoline, diesel, aviation fuels and chemicals. Over time the catalyst material used in the process becomes contaminated with impurities present in feed products and must be removed and exchanged with new or reconditioned material. In many of these catalytic processes the impurities contain trace metals and sulfides that may become pyrophoric when exposed to an air atmosphere. To control these conditions a maintenance procedure known as “Inert Entry” is carried out.
It is well known that combustion requires three points on the “Flame Triangle” namely, fuel, heat, and oxygen. Man's early entry into catalytic reactors and the associated downtime reduction is made possible by eliminating the oxygen from the interior of the catalytic reactor vessels with an inert gas, such as nitrogen, thereby preventing combustion and greatly reducing the costly process downtime. Such downtime to the industry is frequently measured in millions of dollars of lost production. Management of the vessel's environmental temperature is additionally necessary to prevent workers from suffering from heat prostration.
The purging operation is normally carried out by providing a continuous maximum flow of nitrogen gas into the interior of the vessel to insure that an inert atmosphere is maintained at all times during the removal and replacement of the catalyst material. Typically, the cryogenic nitrogen trucked to the facility or plant for use in the purging process will be considerably more expensive than the maintenance contractor charges. For example, the cost of nitrogen may be of the order of 1 to 2 million dollars as compared to the contractor's costs, which are typically less than half that amount.
Operations similar to the above are also experienced in the Liquid Natural Gas industry, whereby maintenance is carried out inside gas storage vessels. Often it is neither possible nor practical to remove gas product from vessels and maintenance is possible only by assuring that the vessel is maintained with an inert gas purge to control oxygen ingress and prevent combustion.
Life Support Apparatus manufactured by Breathing Systems, Inc. in Florida, and others, provide breathable air and voice communications to crew members working in nitrogen purged catalytic reactors and other vessels. Each crew member wears a helmet connected to the breathable air supply and to a communications hub via a 100 to 300 foot long umbilical cable. FIG. 1, herein, is a simplified schematic of one such life support apparatus. Referring to that figure, reference numeral 10 represents a centrally located station containing an air monitor and voice communications console 10c, housing a breathing air monitor 10a and a helmet communication amplifier 10b. A primary 10d and secondary 10e source of breathing air, under the control of the console 10c, supply air to the crew member's helmet 14 via a flexible line 11. Duplex voice communications to the crew member's earphones 14a and from the member's microphone 14b are transmitted on electrical conductors 12a-d which form part of an umbilical cable including the air line 11. Conductors 12b and 12d may be considered to be ground wires and conductor 12a may be considered to be a low voltage rail, (say powered at 6 DC volts), providing power to the helmet microphone. These conductors may also serve to accommodate critical parameter sensors, to be described in connection with this invention. Additional helmets 15 are shown below the console. The voice communications not only exist between an individual crew member and an operator at the central station but also between each of the individual crew members.
FIG. 2 is a schematic diagram of a typical prior art operator's voice communication system where the inputs from the crew members microphones are supplied via input summing resistors R12 to a summing amplifier 26, then to an audio band pass filter 26a and ultimately through low power amplifiers 26c to the remote crew member's earphones.
There is a need to improve the safety of personnel working in remotely located hazardous areas or zones which may contain pyrophoric materials or combustible gas by monitoring certain critical parameters, i.e., the oxygen concentration and temperature to which the personnel are exposed. In addition there is need to allow the plant operator to conserve the amount of inert gas, e.g., nitrogen, flowing into the vessel and its cost while still maintaining an adequate inert atmosphere to which the worker inside the vessel is exposed.